RFR: 7903672: Some issues with missing dependency errors

[Jorn Vernee]

- Only error on missing dependencies, if the dependent is not skipped
- Scan element types of arrays for missing dependencies as well
- Don't issue warnings about unsupported types/fields for skipped elements

I've pulled the code that issues errors for missing dependencies into a separate phase (`MissingDepWarner`), since the control flow in some of the visitor methods was kinda tricky to follow if this stayed mixed in with the `IncludeFilter` code. The new phase just cleanly checks whether the visited element has a `Skip` on it and then returns at the start, before doing any work.

I've also re-ordered the phases a bit so that the phases that add Skips run first, so that we don't issue warnings/errors for things that are skipped any way by a later phase. Since `UnsupportedFilter` both adds Skips and issues warnings, it sits right in the middle.

Q: should we also return early in `NameMangler` if an element is skipped? (Right now it's just doing extra work).

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Commit messages:
 - MissingDepWarner + reorder passes
 - add dummy field
 - scan element types of array fields
 - Only error on skipped dependencies, if the dependent is not skipped

Changes: https://git.openjdk.org/jextract/pull/217/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=217&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903672
  Stats: 274 lines in 9 files changed: 246 ins; 24 del; 4 mod
  Patch: https://git.openjdk.org/jextract/pull/217.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/217/head:pull/217

PR: https://git.openjdk.org/jextract/pull/217